1. Field of the Invention
The invention relates to disposable syringes; more particularly, the invention relates to disposable syringes which may be used only once and inherently prevent a second use from occurring.
2. Description of the Related Art
Plastic hypodermic syringes are mass-producible, and are intended to be sterilized or discarded after only one use to prevent the spread of blood-borne diseases such as hepatitis and AIDS from one user to the next. For example, if an individual using a syringe is carrying a blood-borne disease, a subsequent user of the syringe runs a great risk of contracting the disease.
Plastic hypodermic syringes which are not pre-filled with medication are usually loaded by the action of a plunger which creates a reduced pressure in the barrel and results in the intake of fluid into the barrel. Such syringes depend on a rubber seal attached to the plunger to ensure that a reduced pressure can be effected because of the wider tolerances experienced in plastic molding, in contrast to precisely ground glass syringes which do not require a rubber seal.
One example of a syringe that can be used only one time is described in U.S. Pat. No. 5,205,825 to Allison et al., the teachings of which are incorporated by reference herein. The device described by Allison et al. includes a conventional plastic plunger (which is X-shaped in section) and a plastic barrel. A metal clip is positionable between the plunger and the inner wall of the barrel. The clip includes two pairs of barbs. One pair of outer barbs is intended to engage the inner wall of the barrel and the other pair of barbs is intended to engage the vane walls of the plunger. The barbs are angled to allow the plunger to be withdrawn from the barrel for medication loading. The clip stays motionless relative to the barrel when the plunger is withdrawn, because the outer barbs dig into the inner wall of the barrel. When the plunger is pushed back into the barrel to expel the liquid inside, the inner barbs of the clip engage the plunger, and the clip moves with the plunger with respect to the barrel. If one attempts to withdraw the plunger a second time, the clip and plunger remain in place because the outer barbs of the clip again dig into and engage the inner barrel wall, and the distal end of the plunger strikes the immobile clip.
The Allison et al. device suffers from several drawbacks. First, the clip described is difficult to manufacture. Second, proper placement of the clip with respect to the plunger is difficult to gauge by eye. In other words, it is difficult to pre-measure the proper dosage (for a pre-filled syringe) or draw the correct amount of medication into an empty syringe utilizing the Allison et al. clip. Third, during manufacture and assembly of the syringe, it is difficult to ensure that the clip is properly positioned relative to the plunger and barrel because the clip can slip backwardly during placement.
Accordingly, it is an object of the invention to provide a disposable hypodermic syringe which cannot be used more than once.
It is another object of the invention to provide a disposable hypodermic syringe which immobilizes the plunger inside the barrel after a single use.
It is another object of the invention to provide a disposable hypodermic syringe which is easy and inexpensive to manufacture and assemble.
The above and other objects are achieved by the invention which is a hypodermic syringe. The syringe has a barrel and a plunger. The plunger has a distal wall and at least one vane, and the vane is provided with serrations or ratchet teeth. The serrations each have a vertical rear wall and a sloped top edge, the slope angling downward in a forward (distal) direction. A barbed clip is provided that fits on the serrated vane. The barbs on the clip engage the inner wall of the barrel.
When the plunger is withdrawn for drawing medication into the barrel, the barbs of the clip dig into and engage the inner wall of the barrel and the clip remains in position with respect to the barrel. For pre-filled syringes, the plunger does not need to be withdrawn. Because the tops of the serrations are sloped, the ridged/serrated vane of the plunger passes easily under the stationary clip. When the plunger is re-inserted into the barrel, the clip engages one of the vertical rear walls of the serrations and moves along with the plunger deeper into the barrel (distally). If one attempts to re-retract the plunger, the distal wall of the plunger strikes and abuts the clip, and the clip is immobilized because the barbs engage and dig into the inner wall of the barrel.
It is easy to position the clip precisely on the plunger during manufacture of the invention owing to the serrations on one or more of the vanes. Once the clip is positioned, it cannot slip backwardly with respect to the plunger because a) the barbs engage the barrel, and b) the rear vertical walls of the serrations engage the rear portion of the clip and prevent backslip. Further, it is much easier to meter doses of the contents of a syringe because of the prevention of the backslip of the clip and because dose indicators can be printed or formed on the serrations, for example. The inventive clip is also easy to manufacture.